Priming for tolerance and cohesion at replication forks

Genome duplication is coupled with DNA damage tolerance (DDT) and chromatin structural changes. Recently we reported that mutations in Primase subunits or factors that bridge Polα/Primase with the replicative helicase, Ctf4, caused abnormal usage of DDT pathways, negatively influenced sister chromatid cohesion (SCC), and associated with increased fork reversal. (1) We also found that cohesin, which is paradigmatic for SCC, facilitates recombination-mediated DDT. However, only the recombination defects of cohesin, but not of cohesion-defective Polα/Primase/Ctf4 mutants, were rescued by artificial tethering of sister chromatids. Genetic tests and electron microscopy analysis of replication intermediates made us propose that management of single-stranded DNA forming proximal to the fork is a critical determinant of chromosome and replication fork structure, and influences DDT pathway choice. Here we discuss the implications of our findings for understanding DDT regulation and cohesion establishment during replication, and outline directions to rationalize the relationship between these chromosome metabolism processes.